1. Field of the Invention
The present invention relates to a spray-coated roll used in an apparatus for continuously galvanizing steel sheets, and more particularly, to the surface coating structure of the roll.
2. Discussion of Background Information
Galvanized steel sheets (inclusive of zinc-aluminum hot dipped steel sheets) are used as outer body panels for vehicles, corrosion resistant material for building or the like, and are manufactured mainly by an apparatus as shown in FIG. 1.
That is, a steel sheet 1 that is to be galvanized is first annealed in a continuous annealing furnace, then, the steel sheet, guided by a turn down roll 11, is passed through a snout 2 maintained in a reducing atmosphere and then introduced into a ceramic pot 10 containing the galvanizing bath 3, where the steel sheet 1 is galvanized while passing along a sink roll 4, a front support roll 5 and a back support roll 6. Thereafter, the galvanized steel sheet is passed through wiping nozzles 7, a touch roll 8 and a top roll 9 to adjust the thickness of the resulting galvanized layer.
In general, the rolls described above, except roll 11, are immersed in the galvanizing bath or are in contact with the high temperature galvanized steel sheet, so that they are required to satisfy the following conditions:
(1) the roll is hardly subject to erosion due to molten metal;
(2) the roll is hardly abraded by contact with the passing steel sheet;
(3) when the roll is taken out of the galvanizing bath for maintenance and inspection, zinc easily peels off of the surface of the roll;
(4) the roll can be used over a long period of time; and
(5) the cost of the roll is low.
In order to provide rolls satisfying these conditions, i.e., rolls used for galvanization, there have hitherto been proposed the following surface coating methods:
(a) a self-fluxing alloy is sprayed onto the surface of the roll as disclosed in Japanese Patent Laid-Open No. 54-69529;
(b) a ceramic such as ZrO.sub.2, Al.sub.2 O.sub.3 or the like is sprayed onto the surface of the roll as disclosed in Japanese Patent Laid-Open Nos. 61-37955 and 61-117260;
(c) a surface coating layer of 0.1-2.4 mm in thickness is formed consisting of at least one of WC, Cr.sub.3 C.sub.2 and TiC and the reminder of hot corrosion-resistant metal or its alloy is disclosed in Japanese Patent Application Laid-Open No. 58-25468; and
(d) a spray-coated layer composed of WC-Co series cermet material containing 5-28 wt % of Co and having a porosity of not more than 1.8% is formed on the roll surface as disclosed in Japanese Patent Laid-Open No. 1-225761.
Among these conventional coated layers, cases (a) and (b) are more resistant than a non-treated steel roll to the galvanization process. However, the self-fluxing alloy layer of (a) or the oxide ceramic layer of (b) tend to locally peel off from the surface of the roll when in use over about 2 weeks. This local peeling of the roll surface transfers an undesired pattern onto the sheet metal and, consequently, the commercial value is lowered considerably.
In case (c), the carbide such as WC, Cr.sub.3 C.sub.2 and TiC or the like shows an excellent resistance to erosion due to the galvanization process, but the coated layer cannot be formed by the spraying process alone. In this regard, the carbide is applied with a metal as a binder to form a so-called cermet coated layer. However, the cermet coated layer has the drawback that the performance is lowered considerably in accordance with the kind of the metal used as the binder, and cannot be put into practical use. That is, when the carbide is mixed with Ni, Si or the like as a heat-resistant metal, the resulting coated layer is rapidly eroded by the galvanizing solution with the resultant loss of function of the coated layer.
Moreover, when the cermet coated layer is made from a carbide applied with Co, it is relatively durable to the galvanizing solution. However, this coating may be eroded in a short time although the cause of erosion is not clear. The latter coated layer is required to have a thickness of not less than 0.1 mm. If the thickness is less than 0.1 mm, the effect is poor.
In case (d), the inventors have proposed a method for improving the carbide-Co cermet coated layer. This method restricts the porosity of the coated layer to not more than 1.8% and thereby, even at a thickness of less than 0.1 mm, the coated layer is sufficiently durable to the galvanizing solution.
Recently, the appearance of the galvanized steel sheet has become more important than improvements to the corrosion resistance, as a performance requirement. That is, a slight cloud of gloss generated in the galvanized surface, a linear transferred damage on the galvanized surface due to groove formed in the sink roll immersed in the galvanizing bath, and the like, which have been ignored in the conventional technique, have recently become important performance criteria.
In the spray-coated layer of WC-Co cermet system, it is often difficult to produce high quality galvanized steel sheets by defining only the amounts of both components used. Moreover, if the resulting WC-Co cermet layer satisfies the requirements, it has a drawback that the life is relatively short.